1. Field of the Invention
The present invention relates to an organic light emitting element having an anode, a cathode, and a layer containing an organic compound in which light emission can be obtained by applying an electric field (hereafter referred to as an xe2x80x9corganic compound layerxe2x80x9d). In general, light emitted when returning to a base state from a singlet excitation state, and light emitted when returning to a base state from a triplet excitation state exist as organic compound light emissions generated by the application of an electric field. In particular, the present invention relates to organic light emitting elements using organic compounds in which light emission is capable of being generated from a triplet excitation state. Note that the term light emitting device in this specification indicates image display devices and light emitting devices using organic light emitting elements as light emitting elements. Further, modules in which a connector, for example an isotropic conductive film (flexible printed circuit, FPC), a TAB (tape automated bonding) tape, or a TCP (tape carrier package) is attached to organic light emitting elements are all included in the category of light emitting devices, as are modules in which a printed wiring board is provided in an end portion of TAB tape or TCP, and modules in which an IC (integrated circuit) is directly mounted to light emitting elements by a COG (chip on glass) method.
2. Description of the Related Art
Organic light emitting elements are elements which emit light by the application of an electric field. The light emitting mechanism is one in which electrons injected from a cathode recombine within an organic compound layer with holes injected from an anode, forming excited state molecules (hereafter referred to as xe2x80x9cmolecular excitonsxe2x80x9d), by the application of a voltage to the organic compound layer sandwiched between the electrodes. Energy is released when the molecular excitons return to a base state, emitting light.
The organic compound layer is normally formed by a thin film having a thickness less than 1 xcexcm for these types of organic light emitting elements. Further, organic light emitting elements are self light emitting elements in which light is emitted by the organic compound layers, and therefore a back light like that used in a conventional liquid crystal display is not necessary. Consequently, the ability to manufacture light emitting elements that are extremely thin with light weight is a big advantage.
Furthermore, the period of time from the injection of a carrier until recombination occurs in an organic compound layer having a thickness on the order of 100 to 200 nm, for example, is on the order of several tens of nanoseconds when considering the carrier mobility of the organic compound layer. Even when including a period required for a process from when the carrier recombines until light is emitted, the light emission can be performed within order of microsecond. The light emitting elements therefore have a fast response speed.
In addition, drive using a direct current voltage is possible because the organic light emitting elements are carrier injecting light emitting elements, and therefore it is difficult for noise to develop. With regard to driving voltage, it has been reported (reference 1: Tang, C. W., and VanSlyke, S. A., xe2x80x9cOrganic Electroluminescent Diodesxe2x80x9d, Applied Physics Letters, Vol. 51, No. 12, pp. 913-5 (1987)) that a sufficient brightness of 100 cd/m2 at 5.5 V was achieved by first taking an extremely thin film of an organic compound layer with a uniform film thickness on the order of 100 nm, selecting an electrode material so as to make the carrier injection barrier with respect to the organic compound layer smaller, and in addition, introducing a hetero structure (two layer structure).
Organic light emitting elements are therefore under the spotlight as next generation flat panel display elements due to their thin size, light weight, high speed response, and driving at D.C. voltage and low voltage. Further, light emitting elements are self light emitting and have a wide angular field of view, and therefore their visibility is comparatively good and they are considered to be effective as elements used in the display screens of portable devices.
It has already been stated that emitted light in organic light emitting elements is a phenomenon in which light is emitted when molecular excitons return to a base state, and it is possible for singlet excitation state (S*) and triplet excitation state (T*) molecular excitons to exist as molecular exciton types formed by organic compounds. Further, the statistical generation ratio in organic light emitting elements is considered to be S*:T*=1:3 (reference 2: Shirato, J., xe2x80x9cMonthly Display Supplement, From Organic EL Display Fundamentals to the Latest Informationxe2x80x9d (TechnoTimes Corp.), p. 28-29).
However, for general organic compounds at room temperature, the emission of light from the triplet excitation state (T*) is not observed, and normally only light emitted from the singlet excitation state (S*) can be observed. The base state of organic compounds is a singlet base state (S0), and therefore transitions from T* to S0 (phosphorescence process) become prohibited transitions to a considerable degree and transitions from S* to S0 (fluorescence process) become allowed transitions.
In other words, normally only the singlet excitation state (S*) contributes to light emission, and this is the same for organic light emitting elements. However, the theoretical limit of the internal quantum efficiency in organic light emitting elements (the proportion of photons generated with respect to the carrier injected) is 25% based on the fact that S*:T*=1:3.
Furthermore, the light emitted is not all emitted to the outside. A portion of the light cannot be extracted due to the component of the organic light emitting element (organic compound layer materials, electrode materials) and the index of refraction indigenous to the substrate material. The ratio of light extracted to the outside with respect to the light emitted is referred to as a light extraction efficiency. The light extraction efficiency is thought to be on the order of 20% for organic light emitting elements having a glass substrate.
For the above reasons, even if all of the injected carrier are able to form molecular excitons, the theoretical limit of the final ratio of photons extracted to the outside with respect to the number of injected carriers (hereafter referred to as an xe2x80x9cexternal quantum efficiencyxe2x80x9d) is 25%xc3x9720%=5%. That is, even if all of the carrier recombines, only 5% is extracted as light.
However, in recent years organic light emitting elements capable of converting energy emitted when returning to a base state from a triplet excitation state (T*) (hereinafter referred to as xe2x80x9ctriplet excitation energyxe2x80x9d) into an emission light have been announced one after another, their high light emission efficiency grabbing attention. (Reference 3: D. F. O""Brien, M. A. Baldo, M. E. Thompson and S. R. Forrest, xe2x80x9cImproved energy transfer in electrophosphorescent devicesxe2x80x9d, Applied Physics Letters, vol. 74, No. 3, 442-444 (1999)). (Reference 4: Tetsuo TSUTSUI, Moon-Jae YANG, Masayuki YAHIRO, Kenji NAKAMURA, Teruichi WATANABE, Taishi TSUJI, Yoshinori FUKUDA, Takeo MAKIMOTO and Satoshi MIYAGUCHI, xe2x80x9cHigh Quantum Efficiency in Organic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive Center,xe2x80x9d Japanese Journal of Applied Physics, Vol. 38, pp. L1502-L1504 (1999)).
An organometallic complex having platinum as a central metal (hereafter referred to as a xe2x80x9cplatinum complexxe2x80x9d) is used in reference 3, while an organometallic complex having iridium as a central metal (hereafter referred to as an xe2x80x9ciridium complexxe2x80x9d) is used in reference 4. Both organometallic complexes are characterized by their introduction of a tertiary transition element as a metal center. Of course, materials do exist that exceed the theoretical limiting value of 5% for the external quantum efficiency discussed above.
As shown in reference 3 and in reference 4, organic light emitting elements that use organic compounds capable of converting triplet excitation energy into light emission (hereafter referred to as xe2x80x9ctriplet light emitting materialsxe2x80x9d) can achieve an external quantum efficiency that is higher than the conventional one. If the external quantum efficiency becomes higher, then the brightness of emitted light also increases. It is therefore considered that organic light emitting elements using triplet light emitting materials will occupy a large amount of future development as means of achieving high brightness light emission and light emitting efficiency.
However, platinum and iridium are both so-called precious metals, and therefore platinum and iridium complexes using these metals are also high cost, and it is anticipated that this will have a harmful influence on future cost reductions. Further, these metals are both scarce metals, and are therefore difficult to supply for mass production.
The platinum complex and the iridium complex are organometallic complexes in which the central metal and the ligand benzene ring are "sgr"-bonded by direct bonding. A long period of time is required for synthesis, and the yield is poor, and therefore these complexes are not good for productivity. From the standpoint of productivity, Werner complexes such as tris(8-quinolinolate) aluminum (hereafter referred to as xe2x80x9cAlq3xe2x80x9d), often used in organic light emitting elements, are generally thought to be effective.
In addition, the color of light emitted by the iridium complex is green, namely an intermediate wavelength in the visible light region. If the platinum complex is used as a dopant, the light emitted has a red color with relatively good color purity. However, the host material also emits light, and there is a disadvantage in that the color purity becomes poor if the concentration is low. For cases in which the concentration is high, a disadvantage exists in that there is concentration quenching, and therefore the light emission efficiency drops. In other words, high efficiency emission of high color purity red color and blue color light cannot be obtained from organic light emitting elements capable of converting triplet excitation energy into light emission.
Considering the future manufacture of a full color flat panel display using light emission in red, green, and blue colors, it is therefore necessary to achieve mass production using low cost raw materials which have a high external quantum efficiency, similar to that of the platinum complex or the iridium complex, and which give high color purity red color and blue color light emissions.
From the above discussion, other than for existing organic metal complexes using platinum or iridium, there is arisen an indispensable necessity for the development of triplet light emitting materials.
The present invention has been made in view of the above, and an object of the present invention is to provide a triplet light emitting material which is manufactured at lower cost than conventional triplet light emitting materials. Further, another object of the present invention is to provide an organic light emitting element which has a higher light emission efficiency, and can be manufactured at lower cost, than conventional light emitting elements by using the light emitting material of the present invention.
In addition, still another object of the present invention is to provide a low cost light emitting device that is bright and has low electric power consumption by using an organic light emitting element having a high light emission efficiency obtained by implementing the present invention. Still another object of the present invention is to provide electronic equipment using the light emitting device.
The heavy atom effect is well known in the photoluminescence field as a method of converting triplet excitation energy light emission. The heavy atom effect is a phenomenon in which the spin-orbit interaction becomes larger, and phosphorous light emission, a prohibited transition (T* to S0), is promoted by introducing heavy atoms into molecules having light emitting properties, or by placing heavy atoms around the periphery of solvent or the like having a melted light emitting material. Note that the term heavy atom as used here indicates atoms possessing a large atomic nucleus weight (corresponding to atomic number, that is the number of positive electric charges in the atomic nucleus).
Platinum and iridium are heavy atoms which can develop the spin-orbit interaction, and effectively promote phosphorescence. In this respect, the platinum complex and the iridium complex discussed above can be said to be extremely effective triplet light emitting materials.
However, the effectiveness of the heavy atom effect is determined by a spin-orbit coupling coefficient, which is characteristic to each atom, and therefore the atoms used in order to cause the heavy atom effect are very limited. Many raw materials and the like containing these atoms are high cost.
A method of converting triplet excitation energy into light emission without the use of heavy atoms is thus preferable. Conceptually, transitions from a triplet excitation state to a base state should be allowed transitions. In other words, it is thought that triplet excitation energy can be converted into light emissions provided that the base state is a triplet state. For example, the base state becomes a triplet state provided that the highest occupied molecular orbital (HOMO) is degenerate, as with oxygen molecules.
Normally this type of state is not seen in hydrocarbon compounds, but it is possible to form a triplet state in energy levels of the central metal of metallic complexes. Binuclear complexes having paramagnetic metals as central metals (metal complexes having two central metals) can be given as an example.
A phenomenon is seen in binuclear complexes having paramagnetic metals as central metals in which unpaired electrons of the paramagnetic metals often couple within the complex, and a ferromagnetic or antiferromagnetic interaction develops. The electrons are considered to be in a triplet state for the ferromagnetic interaction case. Furthermore, although the electrons are in a singlet state for the antiferromagnetic case, they will be in a triplet state at a certain temperature or above (reference 5: Fundamental Complex Engineering Research Society, xe2x80x9cComplex Chemistry: Fundamentals and Latest Topicsxe2x80x9d (Kodansha), pp. 48-9).
By triplet state electrons thus formed contributing to the emission of light, the transformation of triplet excitation energy into light emission is considered to be possible. The applicants of the present invention have focused upon applying binuclear complexes as light emitting materials for organic light emitting elements.
Furthermore, the applicants of the present invention consider that the total atomic weight becomes very large in accordance with a plurality of central metals being in contact and forming a cluster state, and thus that there is a substantial possibility of causing an effect similar to the heavy atom effect. This is also a reason for focusing upon binuclear complexes.
In addition, the excitation energy state changes in accordance with changing the combination of the central metals if a binuclear complex is used, and therefore it is considered that the color of light emitted can be changed to a certain extent. Namely, there is an advantage in that tuning of the color of light emitted becomes possible without changing the ligands.
From this background, a binuclear complex possessing luminescent ligands is used in the organic light emitting elements in the present invention. Note that, although Werner complexes are used in the present invention due to their ease of synthesis and good productivity, the use of organometallic complexes in which a central metal and carbon atom ligands are directly bonded can be considered to achieve better characteristics. This can be seen strikingly for the iridium complex.
The binuclear complexes used in the present invention can be expressed by general formulae 1 to 4, shown below. 
In general formula 1, M1 and M2 denote bivalent metallic ions or bivalent oxo-metallic ions. Further, X denotes a benzene ring or a fused ring made from a benzene ring, which may have substituents. R1 denotes hydrogen or an alkyl group; and R2 to R11 each denote hydrogen or an alkyl group, and R2 to R11 may all be identical, or may be different.
In general formula 2, M1 and M2 denote bivalent metallic ions or bivalent oxo-metallic ions. R1 denotes hydrogen, an alkyl group, an alkoxyl group, or aryl group. R2 to R11 each denote hydrogen or an alkyl group, and R2 to R11 may all be identical, or may be different.
In general formula 3, M1 and M2 denote bivalent metallic ions or bivalent oxo-metallic ions. R1 denotes hydrogen or an alkyl group. Further, X1 and X2 each denote a benzene ring or a fused ring made from benzene. X1 and X2 may each be identical, or may be different, and each may have substituents.
In general formula 4, M1 and M2 denote bivalent metallic ions or bivalent oxo-metallic ions. R1 and R2 each denote hydrogen, an alkyl group, an alkoxyl group, or aryl group, and R1 and R2 may be identical, or may be different.